I think the answer has to do with calls to action and the assumption users make that the cursor hand is a link to another web page or another part of the site, while not all buttons are hyperlinks. Probably not as much of an argument in these days, though, especially as most sites are single page applications where basically every button does indeed do some sort of reroute, target blank (new tab), or network call at the very least.
Also, to be pedantic, not all buttons are entirely clickable, only small portions of them.
Edit: Interesting that HN doesn't use the cursor hand for reply / updating. ;)